Automatic respiratory drive is provided by the bulbopontine area, whereas wilful drive is by the suprapontine area. Suprapontine control can be learned, for example in speaking or playing an instrument, and because it is learned it competes for the control of the respiratory muscles. Irritant receptors lie between the epithelial cells to detect mechanical stimuli such as smoke. ey are more sensitive to mechanical irritants and are less sensitive to chemical irritants in the central airways, and vice versa for the more peripheral airways. Note that they are also sensitive to histamine and can compound bronchoconstriction. Juxtacapillary (J) receptors are present within the alveolar walls and are consequently in close contact with the capillaries. ey respond to an increase in uid within the lung parenchyma. Carotid and aortic bodies make up the peripheral chemoreceptors. pH is detected only by carotid bodies, whereas both the carotid and aortic bodies detect changes in blood oxygen and carbon dioxide. Central chemoreceptors are located on the surface of the medulla and respond only to changes in PaCO2.